Epitaxial process applying light illumination

ABSTRACT

An epitaxial process applying light illumination includes the following steps. A substrate is provided. A dry etching process and a wet etching process are performed to form a recess in the substrate, wherein an infrared light illuminates while the wet etching process is performed. An epitaxial structure is formed in the recess.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to an epitaxial process, andmore specifically to an epitaxial process applying light illumination.

2. Description of the Prior Art

For decades, chip manufacturers have made metal-oxide-semiconductor(MOS) transistors faster by making them smaller. As the semiconductorprocesses advance to the very deep sub-micron era such as 65-nm node orbeyond, how to increase the driving current for MOS transistors hasbecome a critical issue. In order to improve device performance, crystalstrain technology has been developed. Crystal strain technology isbecoming more and more attractive as a means for getting betterperformance in the field of MOS transistor fabrication. Putting a strainon a semiconductor crystal alters the speed at which charges movethrough that crystal. Strain makes MOS transistors work better byenabling electrical charges, such as electrons, to pass more easilythrough the silicon lattice of the gate channel.

Attempts have been made to use a strained silicon layer, which has beengrown epitaxially on a silicon substrate with a silicon germanium (SiGe)layer disposed therebetween. In this type of MOS transistor, a biaxialtensile strain occurs in the epitaxy silicon layer due to the silicongermanium which has a larger lattice constant than silicon. As a result,the band structure alters, and the carrier mobility increases. Thisenhances the speed performance of the MOS transistors.

As sizes of components shrink, sizes and shapes of epitaxial structuresand distances of epitaxial structures to gates need to be controlledprecisely. Thus, forming desired epitaxial structures has become animportant issue in the semiconductor industry.

SUMMARY OF THE INVENTION

The present invention provides an epitaxial process applying lightillumination, which illuminates an infrared light while the epitaxialprocess is performed, which changes etching rates to different crystalplanes to form a desired epitaxial structure.

The present invention provides an epitaxial process applying lightillumination which includes the following steps. A substrate isprovided. A dry etching process and a wet etching process are performedto forma recess in the substrate, wherein an infrared light illuminateswhile the wet etching process is performed. An epitaxial structure isformed in the recess.

According to the above, the present invention provides an epitaxialprocess applying light illumination, which forms a recess in a substrateby a wet etching process illuminated by an infrared light to changeetching rates to different crystal planes, so that the recess having adesired shape can be formed. Hence, an epitaxial structure formed in therecess can achieve a specific requirement. For instance, as theintegration of integrated circuits increase, sizes of components shrink,and pitches between epitaxial structures become closer. Epitaxialstructures having high depths and narrow widths not only can prevent theepitaxial structures from being too close to each other which would leadto the short channel effect and short circuit, but can also improveproblems about diffusion of doped impurities such as boron in theepitaxial structures.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 schematically depict a three dimensional diagram of anepitaxial process applying light illumination according to an embodimentof the present invention.

FIG. 9 schematically depicts a cross-sectional view of an epitaxialprocess applying light illumination along line AA′ of FIG. 8.

DETAILED DESCRIPTION

FIGS. 1-8 schematically depict a three dimensional diagram of anepitaxial process applying light illumination according to an embodimentof the present invention. A tri-gate MOSFET is shown in this embodiment,but the present invention is not restricted thereto. The presentinvention can also be applied in non-planar transistors such asmulti-gate MOSFETs, or planar transistors.

FIGS. 1-2 show a substrate 110 having a fin structure 112. In thisembodiment, the fin structure 112 is only depicted once, but the numberof fin structures 112 is not restricted thereto. More precisely, asshown in FIG. 1, a substrate 110′ is provided. The substrate 110′ may bea semiconductor substrate such as a silicon substrate, a siliconcontaining substrate, a III-V group-on-silicon (such as GaN-on-silicon)substrate, a graphene-on-silicon substrate or a silicon-on-insulator(SOI) substrate. In this embodiment, the substrate 110′ is a siliconsubstrate, so that an etchant in this embodiment can be applied later toetch the substrate 110′ to form a recess, but this is not limitedthereto.

Then, a patterned hard mask 10 is formed on the substrate 110′ to definethe location of the fin structure 112, which will be formed in thesubstrate 110′. In this embodiment, the patterned hard mask 10 is a dualstructure including an oxide layer 12 and a nitride layer 14, but it isnot limited thereto. Thereafter, an etching process P1 may be performedto form the fin structure 112 in the substrate 110′, as shown in FIG. 2.Thus, the fin structure 112 located on the substrate 110 is formedcompletely. In this embodiment, the patterned hard mask 10 is removedimmediately after the fin structure 112 is formed, and a tri-gate MOSFETcan be formed in the following processes. There are three contact facesbetween the fin structure 112 and the following formed dielectric layerfunctioning as a carrier channel whose width is wider than a channelwidth in a conventional planar MOSFET. When a driving voltage isapplied, the tri-gate MOSFET produces a double on-current compared tothe conventional planar MOSFET. In another embodiment, the patternedhard mask 10 is reserved to form a fin field effect transistor (FinFET), which is another kind of multi-gate MOSFET. Due to the patternedhard mask 10 being reserved in the fin field effect transistor, thereare only two contact faces between the fin structure 112 and thefollowing formed dielectric layer.

The present invention can also be applied to other semiconductorsubstrates. For example, a silicon-on-insulator substrate (not shown) isprovided, and then a single crystalline silicon layer being a top partof the silicon-on-insulator substrate (not shown) is etched till anoxide layer being a middle part of the silicon-on-insulator substrate(not shown) is exposed, meaning the fin-shaped structure formed on thesilicon-on-insulator substrate (not shown) is finished.

As shown in FIG. 3, an isolation structure 120 may be formed on thesubstrate 110 beside the fin structure 112. The isolation structure 120may be a shallow trench isolation (STI) structure, which may be formedby processes such as depositing and back-etching, for example beingdisposed on a surface of the substrate 110 other than the fin structure112, but this is not limited thereto.

As shown in FIG. 4, a gate 130 may be formed to cover a part of theisolation structure 120 and disposed across the fin structure 112. Themethod of forming the gate 130 may include the following steps: a gatedielectric layer 132 is formed to cover part of the isolation structure120 and across the fin structure 112; a gate electrode layer 134 coversthe gate dielectric layer 132; a cap layer 136 covers the gate electrodelayer 134; the cap layer 136, the gate electrode layer 134 and the gatedielectric layer 132 are patterned; and a spacer 138 is formed besidethe gate dielectric layer 132, the gate electrode layer 134 and the caplayer 136. In one case, the gate dielectric layer 132 may be siliconoxide, silicon nitride, silicon oxynitride or metallic oxide having ahigh dielectric constant, such as the group selected from hafnium oxide(HfO₂), hafnium silicon oxide (HfSiO₄), hafnium silicon oxynitride(HfSiON), aluminum oxide (Al₂O₃), lanthanum oxide (La₂O₃), tantalumoxide (Ta₂O₅), yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂), strontiumtitanate oxide (SrTiO₃), zirconium silicon oxide (ZrSiO₄), hafniumzirconium oxide (HfZrO₄), strontium bismuth tantalite (SrBi₂Ta₂O₉, SBT),lead zirconate titanate (PbZr_(x)Ti₁-xO₃, PZT) and barium strontiumtitanate (Ba_(x)Sr₁-xTiO₃, BST). The gate electrode layer 134 may be ahighly doped polysilicon, metallic silicon oxide, or a metal gate suchas a metallic silicon oxide, titanium, tantalum, titanium nitride,tantalum nitride or tungsten applying a gate-first process. In oneembodiment, as the gate electrode layer 134 of the gate 130 is apolysilicon electrode layer, a gate-last process may be applied, whichperforms a replacement metal gate (RMG) process to replace thepolysilicon electrode layer with a metal electrode layer. The cap layer136 may be a single layer structure, a multilayer structure composed ofsilicon nitride or silicon oxide. The spacer 138 may be composed ofsilicon nitride or silicon oxide, and the spacer 138 may be a singlelayer structure or a multilayer structure including an inner spacer andan outer spacer. The methods of forming the gate 130 are well known inthe art, and are not described herein.

As shown in FIG. 5, a dry etching process P2 is performed to formrecesses R1 in the fin structure 112 beside the gate 130. The dryetching process P2 may have fluorine gas and chlorine gas imported toetch the silicon fin structure 112.

As shown in FIG. 6, a wet etching process P3 is performed to formrecesses R2 in the fin structure 112 for later formed epitaxialstructures in the recesses R2. The recesses R1 can be enlarged,modified, shape changed or have their surface roughness improved throughperforming the wet etching process P3, so that the recesses R2 havingsmooth surfaces for later formed epitaxial structures or buffer layerscan be formed easily. It is emphasized that an infrared light mustilluminate during the wet etching process P3 to form the recesses R2having diamond-shaped cross-sectional profiles, wherein the depth d/thedistance of the tip t to gate 130 of the diamond-shaped cross-sectionalprofile is preferably less than 25. The diamond-shaped cross-sectionalprofile may include a polygonal cross-sectional profile such as arectangular, hexagonal or octagonal cross-sectional profile. In apreferred embodiment, the depth d/the distance of the tip t to gate 130of the diamond-shaped cross-sectional profile is 10. In this way, as thesize of a component shrinks and pitches between the fin structures 112become closer, epitaxial structures having high depths and narrow widthscan be formed. Hence, the epitaxial structures beside the gate 130 cannot only be prevented from being too close to each other which wouldlead to the short channel effect and short circuit, but problemsregarding diffusion of doped impurities such as boron in the epitaxialstructures can also be improved.

In a preferred embodiment, the wavelength of an infrared lightillumination during the wet etching process P3 is less than 1.2micrometers or within a range of 6˜50 micrometers. Still preferably, thewavelength of an infrared light illumination during the wet etchingprocess P3 is 0.85 micrometers for forming the desired recesses R2 inthe silicon fin structure 112. In an aspect, as an infrared lightilluminates during the wet etching process P3, the etching rate of thewet etching process P3 can be changed, and thus the etching rate of thewet etching process P3 to the crystal plane [100] can be different fromthat to the crystal plane [111]. In this embodiment, the etching rate ofthe wet etching process P3 to the crystal plane [100] is larger thanthat to the crystal plane [111] for forming the recesses R2. Moreprecisely, the etching rate ratio of the wet etching process P3 to thecrystal plane [100]/crystal plane [111] is preferably larger than 2; theetching rate ratio of the wet etching process P3 to crystal plane[100]/crystal plane [111] is still preferably larger than 2.27.

In a preferred embodiment, the etchant of the wet etching process P3 isan organic etchant, such that the wet etching process may be atetramethylammonium hydroxide beilstein (TMAH) etching process, but itis not limited thereto. In another aspect, as the dry etching process P2has fluorine gas and chlorine gas imported, the etchant of the wetetching process P3 is preferably an alkaline etchant to remove acidresidues transforming from the fluorine and chlorine gas, but this isnot restricted thereto.

As shown in FIG. 7, buffer layers 142 may cover the recesses R2. Thebuffer layers 142 may be silicon epitaxial structures or doped epitaxialstructures. The buffer layers 142 may conformally cover the recesses R2to preserve the diamond-shaped cross-sectional profiles of the recessesR2. In addition, the buffer layers 142 may further smooth surfaces ofthe recesses R2.

As shown in FIG. 8, a selective epitaxial growth (SEG) process isperformed to form epitaxial structures 144 in the recesses R2 and on thebuffer layers 142. Since each of the recesses R2 has a diamond-shapedcross-sectional profile, each of the epitaxial structures 144 inherentlyhas a diamond-shaped cross-sectional profile as well. Furthermore, thedepth d/the distance of the tip t to gate 130 of the diamond-shapedcross-sectional profile is preferably less than 25. Still preferably,the depth d/the distance of the tip t to gate 130 of the diamond-shapedcross-sectional profile is 10. In this way, as a size of a componentshrinks and pitches between the fin structures 112 become closer, theepitaxial structures 144 having high depths and narrow widths can beformed. Hence, the epitaxial structures 144 beside the gate 130 not onlycan be prevented from being too close to each other which would lead tothe short channel effect and short circuit, but problems related todiffusion of doped impurities such as boron in the epitaxial structures144 can be improved.

In this embodiment, the epitaxial structures 144 may be silicongermanium epitaxial structures suitable for forming a PMOS transistor,but this is not limited thereto. In another embodiment, the epitaxialstructures 144 may be silicon carbide epitaxial structures or siliconphosphorous epitaxial structures suitable for forming an NMOStransistor. For clarifying the present invention, FIG. 9 schematicallydepicts a cross-sectional view of an epitaxial process applying lightillumination along line AA′ of FIG. 8. The structure shown in FIG. 9 canbe fabricated by the steps illustrated in FIGS. 1-8, wherein the bufferlayers 142 and the epitaxial structures 144 are in the fin structure112. The epitaxial structures 144 (or the buffer layers 142 and theepitaxial structures 144) have diamond-shaped cross-sectional profiles,wherein the depth d/the distance of the tip t to gate 130 of thediamond-shaped cross-sectional profile is less than 25; preferably, thedepth d/the distance of the tip t to gate 130 of the diamond-shapedcross-sectional profile is 10.

Before the recesses R1 are formed and after/while the epitaxialstructures 144 are formed, a single or multiple ion implantationprocesses and/or in-situ doped processes may be performed to dopeimpurities in the epitaxial structures 144, to form a lightly dopedsource/drain and a source/drain of a transistor or to enhanceconductivity via doping impurities such as boron or phosphorous ions.

Accordingly, a non-planar transistor is presented in this embodiment,but the present invention can also be applied in a planar transistor. Aninfrared light can be applied while a wet etching process is performedto form recesses in a substrate beside a gate, for forming epitaxialstructures therein. Methods of forming the recesses and the epitaxialstructures of a planar transistor are similar to those for a non-planartransistor, and thus are not described again.

To summarize, the present invention provides an epitaxial processapplying light illumination, which forms a recess in a substrate by awet etching process illuminated by an infrared light to change etchingrates for different crystal planes so the recess having a desired shapecan be formed. Hence, an epitaxial structure formed in the recess canachieve a specific requirement. For instance, as the integration of theintegrated circuits increases, sizes of the components will shrink andpitches between epitaxial structures become closer; the resultantepitaxial structures having high depth and narrow width not only mayprevent the epitaxial structures from being too close to each other,thereby also preventing the short channel effect and short circuiting,but may also improve problems related to diffusion of doped impurities,such as boron, in the epitaxial structures.

The wavelength of an infrared light for illumination while the wetetching process is performed is less than 1.2 micrometers or within arange of 6˜50 micrometers. Still preferably, the wavelength of theinfrared light is 0.85 micrometers. Thereby, the etching rate of the wetetching process to the crystal plane [100] can be different from that tothe crystal plane [111]. In this embodiment, the etching rate of the wetetching process to the crystal plane [100] is larger than that to thecrystal plane [111] to form the recesses. More precisely, the etchingrate ratio of the wet etching process to the crystal plane [100]/crystalplane [111] is preferably larger than 2, and more preferably is largerthan 2.27.

The etchant of the wet etching process may be an organic etchant, suchthat the wet etching process may be a tetramethylammonium hydroxidebeilstein (TMAH) etching process. Furthermore, before the recesses areformed by the wet etching process, a dry etching process may beperformed to form pre-recesses. Due to the dry etching process importingfluorine and chlorine gas to etch a silicon substrate, the etchant ofthe wet etching process is preferably an alkaline etchant to furtherremove acid residues transformed from the fluorine and chlorine gas.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. An epitaxial process applying light illumination,comprising: providing a substrate, wherein the substrate has at least afin structure; forming a gate on the substrate; performing a dry etchingprocess and a wet etching process to form a recess having adiamond-shaped cross-sectional profile in the substrate beside the gate,wherein an infrared light illuminates while the wet etching process isperformed to change etching rates for different crystal planes, to formthe recess having only one tip pointing toward a gate channel of thegate, and the recess is located in the fin structure; and forming anepitaxial structure in the recess.
 2. The epitaxial process applyinglight illumination according to claim 1, wherein the dry etching processcomprises imported fluorine gas and chlorine gas.
 3. The epitaxialprocess applying light illumination according to claim 2, wherein thewet etching process comprises an alkaline etchant.
 4. The epitaxialprocess applying light illumination according to claim 1, wherein thewet etching process comprises an organic etchant.
 5. The epitaxialprocess applying light illumination according to claim 4, wherein thewet etching process comprises a tetramethylammonium hydroxide beilstein(TMAH) etching process.
 6. The epitaxial process applying lightillumination according to claim 1, wherein the wavelength of theinfrared light is less than 1.2 micrometers or within a range of 6˜50micrometers.
 7. The epitaxial process applying light illuminationaccording to claim 6, wherein the wavelength of the infrared light is0.85 micrometers.
 8. The epitaxial process applying light illuminationaccording to claim 1, wherein the etching rate of the wet etchingprocess to a crystal plane [100] is different from that to a crystalplane [111].
 9. The epitaxial process applying light illuminationaccording to claim 8, wherein the etching rate of the wet etchingprocess to crystal plane [100] is larger than that to crystal plane[111].
 10. The epitaxial process applying light illumination accordingto claim 9, wherein the etching rate ratio of the wet etching process tocrystal plane [100]/crystal plane [111] is larger than
 2. 11. Theepitaxial process applying light illumination according to claim 10,wherein the etching rate ratio of the wet etching process to crystalplane [100]/crystal plane [111] is 2.27.
 12. The epitaxial processapplying light illumination according to claim 1, wherein the epitaxialstructure has a diamond-shaped cross-sectional profile.
 13. Theepitaxial process applying light illumination according to claim 12,wherein a depth of the epitaxial structure/a distance of a tip of theepitaxial structure to gate is less than
 25. 14. The epitaxial processapplying light illumination according to claim 13, wherein a depth ofthe epitaxial structure/a distance of a tip of the epitaxial structureto gate is
 10. 15. The epitaxial process applying light illuminationaccording to claim 1, wherein the substrate comprises a siliconsubstrate.
 16. The epitaxial process applying light illuminationaccording to claim 1, wherein the epitaxial structure comprises asilicon germanium epitaxial structure.
 17. The epitaxial processapplying light illumination according to claim 1, further comprising:forming a buffer layer covering the recess before the epitaxialstructure is formed.